We have discovered that Pseudomonas aeruginosa specific bacteriophage are present in high concentrations in the sputum of patients with active bronchopulmonary P. aeruginosa infections. At least 12 different phage have been isolated from sputum of eight patients, seven of whom had cystic fibrosis; these phage were characterized by host range sensitivity and electron microscopic morphology. We have suggested that phage might play a role in the natural history of human Pseudomonas infections.